Providing water in remote locations is often quite difficult. For example, soldiers in the field require between 1.5 and 7 gallons of water per day for drinking, washing, and food preparation. Supplying this water to widely distributed ground troops presents a significant logistical burden to the U.S. Military. In some instances, soldiers can obtain water from local water supplies (e.g., civilian supplies, rivers, and lakes), but in cases where no local water is available or where it is potentially contaminated, trucks, helicopters, and other vehicles deliver water to the forward-deployed soldiers. Similar logistical problems face non-governmental organizations performing relief work in remote areas.
The logistical burden of water delivery could be mitigated if soldiers could instead produce their own water directly from water vapor in ambient air. Atmospheric air contains water at concentrations typically between 0.003 and 0.03 kg of water vapor per kg of air. Extraction of logistically significant quantities of water from the air can require processing large air volumes. However, the potential reduction in the burden of delivering water to soldiers in the field makes extraction of water from air worthy of consideration.
Numerous techniques have been developed to obtain potable water in remote locations. One basic technique is the “solar still” in which a clear barrier is extended over a source of moisture (a pit dug into the soil or a source of non-potable water), solar radiation is used to evaporate water from the source, and potable water is condensed and collected from the underside of the barrier. This technique has limited applications since it cannot produce large amounts of potable water and depends on both solar energy and a vaporizable source of moisture. The utility of solar stills has generally been limited to emergency or survival situations.
Another technique has been to condense moisture in the air by forcing moisture-laden air over a refrigerated coil with a fan and collecting the condensed water. This method has typically been used by dehumidifiers, but suffers from the relative inefficiency of the refrigeration cycle as well as the growth of contaminants on the exposed condensation surfaces. For further details, refer to the examples of U.S. Pat. No. 6,755,037 and U.S. Pat. No. 6,588,225.
A further method for extracting water from air is to compress the air to the point where water vapor condenses to form liquid water. This method typically requires large amounts of energy and equipment involving many moving parts including seals that must withstand high pressures. The cost and complexity of this method makes it unattractive. For further details, refer to the examples of U.S. Pat. No. 6,453,684, U.S. Pat. No. 6,360,549, and U.S. Pat. No. 6,230,503.
Yet another technique, likewise used for both dehumidification and water production, has been the extraction of water from air via adsorption with a desiccant. Some of these desiccant systems used to produce potable water use liquid desiccant, which require complicated controls. For example, refer to U.S. Pat. No. 6,156,102. Other such systems use a fixed desiccant, such as silica gel or zeolite, in a batch process. These systems are limited in that the batch process limits the time the systems are used. For example, refer to U.S. Pat. No. 4,344,778, U.S. Pat. No. 4,342,569, U.S. Pat. No. 4,219,341, and U.S. Pat. No. 4,146,372. To overcome this limitation, some water producing systems have used plural desiccant beds in an alternating batch process. For further details, refer to the example of U.S. Pat. No. 4,304,577.
While rotating desiccant wheels have been more commonly employed in dehumidifying air conditioning systems that require continuous operation, the desorbed water is left in the waste streams in these systems. For further details, refer to the examples of U.S. Pat. No. 6,099,623, U.S. Pat. No. 5,931,015, U.S. Pat. No. 5,709,736, U.S. Pat. No. 5,526,651, U.S. Pat. No. 5,242,473, U.S. Pat. No. 5,170,633 and U.S. Pat. No. 3,844,737.
It has been proposed to use a rotating desiccant wheel for the production of liquid water from moisture in the air. Such a system adsorbs water from an incoming air stream on a portion of an intermittently rotated desiccant wheel. The wheel rotates to align with a desorbing section in which a recirculating air supply is heated with an electric heater, passed through the wheel to desorb water and regenerate the desiccant, and then passed over a condenser to condense liquid water. The energy requirements of the condenser and the heater in this system limit its efficiency and utility. For further details, refer to the example of U.S. Pat. No. 4,365,979.